School Email Exchange

How do you isolate the variables that could account for plastic at different depths?

Posted on November 07 2012

Question submitted by Shenendehowa High School

The location of plastic in the water column at any given time is a balance between its buoyancy and the energy that could move the plastic vertically.  The plastics that we collect at or near the sea surface are all buoyant in seawater; that is, the density of the plastic material is less than the density of seawater.  The density of seawater is a function of temperature, salinity, and pressure, and density increases with depth in the ocean.  Therefore, the plastic will naturally float to the surface in the absence of any mechanical force that could move it.

Vertical currents in most places in the ocean are very, very small, thus they are negligible when we consider the energy available to push plastics down into the water column.  The main source of energy creating vertical motion in the upper layers of the ocean is the wind.  The wind can mix surface water to depths of tens of meters in the subtropics.  This occurs in the "wind-mixed layer" of the surface ocean.

We hypothesize that the plastics found in the upper tens of meters of the water column have been pushed away from the sea surface because of the energy of the wind, which creates turbulent vertical motion.  Therefore, to test our hypothesis the only variable we measure is wind speed, which is a measure of the amount of energy available to mix the plastic pieces down.  We then use the MOCNESS, a system of nets that allows us to measure the amount of plastic at discrete depths (e.g., 1, 5, or 10 meters depth) without contaminating the sample with water between the surface and that depth.

Looking like black smoke in the water, the MOCNESS samples for plastics down to 10 meters depth.